Analysis of a gas mixture is facilitated when the mixture is separated into various fractions before analysis proper. Illustratively a sample of respiratory air is prepared for mass-spectroscopy analysis in that the person from whom the alveolar air, that is the air from the actual lung, is to be investigated, is made to inflate a collecting pouch holding about 1 ltr.
Prior to being introduced into the ion source of the mass-spectrometer, the breath sample must be prepared physically. So far and illustratively this has been done using a cold-trap. The respiratory air to be analyzed first arrives at a trap cooled with frozen methanol wherein the water vapor is frozen out and then this air passes into a second trap cooled with liquid nitrogen where the carbon dioxide is frozen out. At this location the gas components that did not freeze, in particular nitrogen, oxygen and noble gases, are removed using a vacuum pump. Next the second cold trap is heated to allow the frozen carbon dioxide to evaporate, and it is then moved through an intake system into the ion source of mass spectrometer.
Especially with respect to routine use on a large scale, the above described method of sampling, storage and preparation incurs substantial drawbacks:
Sampling entails a substantial risk of handling errors. PA1 Only a small quantity of air representing a fraction of the exhalation is sampled; however, the marked carbon dioxide does not necessarily accumulate in the lung in a uniform manner in time and space; in this respect the above method fails to be integrating. PA1 Even though only one random sample has been taken, the sample vessel of 50 ml remains fairly large; this entails space problems if, for instance, eight samples are taken per patient and at the same time samples from several patients must be shipped and stored. PA1 The system requires liquid nitrogen which as a rule is unavailable in medical labs and therefore entails special costs of operation. PA1 The system of cold traps demands careful maintenance and is not easily automated; ice formation at different sites makes is susceptible to malfunctions; in the event of malfunction, the entire analytical the system of cold traps is capital-intensive. PA1 Sample preparation is time-consuming.